IACR-Rothamsted, Harpenden, Herts. AL5 2JQ, UK

Pathogen dispersal can often be a limiting phase in the development of plant disease epidemics. Environmental factors play a vital role in the dispersal process of most pathogens, either directly as the dispersal agent (e.g. wind and rain) or indirectly through their effects on the behaviour of the pathogen vector (e.g. insects). This paper will deal exclusively with the direct influence of environmental factors on dispersal process, with particular reference to fungal pathogens.

Propagule dispersal can be considered as consisting of three phases: removal (take-off), dispersal and deposition (landing). All three phases are influenced by environmental factors. Spores of some fungal pathogens are removed from lesions by specific release mechanisms triggered by environmental cues such as humidity, temperature or light. Release of ascospores of Sclerotinia sclerotiorum and Pyrenopeziza brassicae from turgid asci can occur in response to a decrease in relative humidity. Drying is also responsible for the release of conidia of Peronospora tabacina. Spores of many species are simply blown or shaken off their hosts by the action of wind. To remove spores, the aerodynamic forces acting on them must be greater than the forces holding them to the surface. Since mean wind speeds encountered in plant canopies are general smaller than those needed to remove spores, it seems likely that wind gusts (turbulence) play an important role in this removal mechanism. Rain or spray irrigation can remove spores and pathogenic bacteria from crop surfaces by washing them off in run-off water or by shaking caused by the impact of raindrops. Wind-dispersed spores of Puccinia arachidis are removed in large numbers during rain. Spores of many plant pathogens are held in a mucilage which prevents dispersal by wind and are removed in splash drops or run-off when the mucilage is dissolved by water. Splash drops can be thrown more than a metre from the point of splash, but most travel only a few centimetres. However, small splash droplets have the potential to travel greater distances. The type of rainfall can influence the effectiveness of splash dispersal. The greater the proportion of large raindrops (>3 ;mm) the more effective the rain in spreading pathogens by splash.

Wind conditions within and above crops are highly turbulent. This causes individual propagules to follow different paths and travel different distances, even if released from the same source under the same wind conditions. Therefore, as the propagule plume is dispersed downwind of the source its concentration in the air decreases. This decrease in concentration is often referred to as a concentration gradient. The shape of the gradient depends on the aerodynamic properties of the propagule, the speed and the turbulence of the wind and the nature of the terrain. Increasing turbulence tends to make gradients steeper by more effective vertical mixing. Gradients within crops are much steeper than those above crops because of lower wind speeds and deposition on to the plants, and gradients for splash dispersed pathogens are steeper than those for wind dispersed pathogens.

Deposition of propagules onto host surfaces can be thought as a combination of settling and impaction. The rate of settling onto surfaces depends on the fall speed of the particle while impaction depends the wind speed and the size of the impaction surface as well as on the aerodynamic properties of the particle. Wind gusts can enhance the inertial impaction of particles, especially in crops where mean wind speeds are small.

This paper will use examples, mainly drawn from foliar pathogen systems, to illustrate the main environmental influences on plant pathogen dispersal and will also consider areas for future research.